A number of forms of eye surgery including lamellar corneal surgery, keratomileusis, epikeratophakia, cataract surgery, penetrating keratoplasty, corneal transplantation radial keratotomy as well as laser refractive keratectomy involve a consideration of corneal surface topography. In radial keratotomy, for example, a number of cuts are made into the cornea in order to change its curvature and correct refractive power so that images focus closer to the retina, if not upon it for best visual acuity. It has been reported that after radial keratotomy "about 55 percent of the patients function without glasses and the remaining 45 percent have some degree of improvement." Origination of the technique of radial keratotomy and other techniques in refractive surgery are generally credited to Dr. Svyatasklav Fyodorov of the Soviet Union who is reputed to have performed thousands of such operations.
While ophthalmic surgery is often successfully performed, the results obtained have been subject to variation occasioned by the particular operating "style" of the individual surgeon which dictates the number, location and depth of incision. Elements of subjective judgment are paramount. It would be useful to provide a device that could assist the surgeon in more quantitatively assessing pre-operative and post-operative corneal contours.
The present system relates to improvements in the art of photokeratometry and more particularly to the use of digital image processing techniques to ascertain the radius of curvature, refractive power and contour of the cornea. A keratometer is an instrument for determining the curvature shape of the corneal surface which generally uses a Placido or other illuminated target that is centered around the patient's line of sight. The reflection of a Placido or other illuminated target by the patient's cornea or by the tear film on the anterior surface of the cornea is subsequently analyzed to determine the surface contour of the eye.
The technique in modern form dates from the early thirties when the Zeiss optical company of Germany introduced a "Photo Keratoscope". In general, the art has required the image reflected by the eye to be photographed and the image on the film measured in a second step to derive the quantitative data from which the contour map is generated.
Recent improvements have been in the area of automating this photogrammetric analysis by re-imaging the photograph with a television apparatus and digital signal conversion. After digitization, computer analysis of the resultant information is performed with conventional image analysis algorithms. This type of data analysis is computer intensive and the image formed by the television system contains a large amount of redundant and extraneous information. For adequate resolution the sampling rate must exceed the data frequency by at least three to one, thus generating a huge number of data points for mathematical analysis. Consequently the systems are costly, complex, slow and often lack real resolution in the image analysis. Other means have been used for clinical measurements such as direct casting of the eye surface in plastic or wax and coating the cornea with talcum powder and projecting a grid on this surface for photogrammetric analysis.
The initial development in keratometry came from Gullstrand in 1896. Gullstrand disclosed the foundation for the current technology but his apparatus had no provision to compensate for aberrations in the optical system other than limiting the photographic coverage of the cornea to a 4 mm area. As a result, multiple exposures and calculations were necessary to map the corneal surface. Much of the modern technique was developed by Amsler in 1930 and embodied in his "Photo-Keratoscope" which also required measurement and calculation as a separate step to derive the corneal shape data.
At present, the clinical standard is the Bausch and Lomb Keratometer, which is sold commercially. The Bausch and Lomb Keratometer only measures the average of the corneal radius in two meridians of the central 3 mm "cap" of the cornea. The standard technology does not provide total surface topography of the cornea and thus is inadequate for many diagnostically significant abnormalities, contact lens fitting, or the needs of ophthalmic surgical procedures. In addition, the prior art technique is cumbersome and involves great potential for error.
The standard instrument which is in most common use for central optical zone shape measurement is the Bausch and Lomb Keratometer. Several companies offer similar devices with similar principles of operation. In these devices a single Mire image is projected on a small central portion of the anterior surface of the cornea usually 3 mm in diameter. The user is required to operate several controls to bring the optically split Mire images reflected from the cornea simultaneously into focus and alignment. In addition, the operator manually records the data obtained at two perpendicular axes. Other instruments are also available, such as the Haag-Streit Javal Schiotz device which measures only one axis at a time, but is slightly easier to use and tends to be more accurate in practice than the Bausch and Lomb system. In addition there exists a photographic system made by International Diagnostic Instrument Limited under the trademark "CORNEASCOPE" (and a similar system made by Nidek in Japan), as well as autokeratometers by several manufacturers. The CORNEASCOPE produces instant photographs of the reflection of a Placido disc and requires a second instrument separate from the camera assembly to analyze the data. This system is fairly accurate, but expensive and tedious to use. The autokeratometers all are limited to a single zone of approximately 3 mm diameter and, in cases where the magnitude of the astigmatism is low, are inaccurate in their assessment of axes of astigmatism. Also available are three computer-direct systems which use conventional image analysis algorithms in conjunction with a mini-computer. These are the Corneal Modeling System (CMS) introduced in 1987 by Computed Anatomy, Inc. of New York, N.Y. and the ECT-100, introduced into the market by Visioptic of Houston, Tex. and a system using light emitting diodes disposed in concentric rings built by Zeiss of Germany. The Placido disc-photo technique is superior to the Bausch and Lomb Keratometer because of the much greater amount of corneal surface analyzed from the Placido reflection as opposed to the mires of the Keratometer.
A number of patents have been issued that relate to keratometers. U.S. Pat. No. 3,797,921 proposes the use of a camera to record the Placido reflection from a patients eye. From this photograph, the radius of surface curvature of the cornea is determined at several points and calculated using a complex computer system. The use of a ground glass focusing screen with the small aperture of the optical system and large linear magnification makes use difficult and requires a darkened room for operation.
U.S. Pat. No. 4,440,477 proposes a method and device for measuring the corneal surface, comprising a slit lamp for illuminating the corneal surface, a camera for recording the reflection from the corneal surface, and a processor to calculate the image distance and the radius of curvature of the eye. The operation of the processor is not detailed in U.S. Pat. No. 4,440,477.
A more recent entry into the market is the "Corneal Modeling System" manufactured by Computed Anatomy Incorporated of New York which uses a light cone Placido target in conjunction with a "frame grabber" to digitize and store for conventional image analysis the pictorial data. The Placido is in cylindrical form and illuminated from one end. This cylindrical Placido maintains a small aperture optical system creating a large depth of field of focus for the imaging system and, consequently, requires a sophisticated focus determining apparatus to assure accurate and reproducible image evaluation. This system is said to produce corneal thickness data using a scanning laser, as well as the surface contour but is very expensive and does not lend itself to clinical applications which are increasingly cost driven.
The prior art systems discussed above tend to be both expensive and difficult to use. Many of the prior art devices have a significant potential for error, due to complexity of the calculation, the imaging of the corneal surface and the difficulty in operating these systems.
Since even a normal human cornea will not be perfectly spherical, the illuminated rings will generally be reflected from the corneal surface as a pattern of shapes variously distorted from the circular. The data pertaining to the coordinates of points in the two-dimensional video image is processed to define a three-dimensional corneal surface yielding the equivalent spherical radius of curvature (or dioptric power) for each of the acquired points.